1. Field of the Invention
The present invention relates to the field of photoimaging, and in particular to the base resins of dissolution inhibition resists which are the most common imaging material of the semiconductor device industry.
2. Description of Related Art
Dissolution inhibition resists, particularly those based on novolak resins, are currently widely employed for imaging semiconductor devices using ultraviolet (UV) light These resists comprise 1) a base resin and 2) a photoactive component (PAC), the latter of which is also termed an inhibiting additive when present in this type of resist The base resin is typically a phenolic resin, and the photoactive component is typically a diazonaphthoquinone. Dissolution inhibition resists are known to the art. See, for example, Photoreactive Polymers, The Science and Technology of Resists, Chapter 5, by Amost Reiser, Wiley-Interscience, John Wiley & Sons, New York (1989).
Such typical dissolution inhibition photoresists are based on two phenomena: the inhibition by diazdnaphthoquinones of the dissolution of phenolic resins in aqueous alkali, and the photochemical transformation of the diazonaphthoquinones into indene carboxylic acids. ##STR1##
The phenolic resin is usually, but not always, a phenol-formaldehyde polycondensate of low molecular weight known as a novolac. On their own, these resins are readily soluble in dilute aqueous alkali, but when a suitable diazoquinone derivative is added, their rate of dissolution is greatly reduced. Nonionizable hydrophobic diazoquinone acts here as a dissolution inhibitor. On irradiation, the diazoquinone transforms into an acid that is soluble in aqueous alkali and which is no longer a dissolution inhibitor and may even promote the dissolution of the resin. In practice, a mixture of novolac and, say, 15% of a diazonaphthoquinone is coated as a thin film on a substrate, for example a silicon wafer, the film is exposed to a radiation pattern and subsequently treated with dilute alkali. The exposed areas of the resist dissolve much faster than the unexposed ones; if the process is stopped in time, a patterned resist film remains in the unirradiated areas. Such material functions as a positive photoresist.
The lithographic contrast achievable with dissolution inhibition resists depends not only on the inhibiting additive (the photoactive component or PAC), but also on the phenolic base resin in which the PAC is dispersed. The fundamental property of a resin which renders it suitable for use in microlithography is the ability of the resin to have its dissolution in aqueous base inhibited by the addition of various diazonaphthoquinones (DNQ) or other active additives. This property is commonly referred to as its "inhibitability." It is this property which determines the image contrast of the resist made from a particular base resin. Image contrast and resin inhibitability are thus intimately linked.
JP 8 337 616 A discloses a chemically-amplified positive working photoresist composition and does not disclose compositions or related topics for dissolution inhibition photoresists apart from certain dissolution control agents being disclosed as optional additives in these chemically-amplified systems. Such modified chemically-amplified systems, even though they may contain dissolution control agents, are very distinct and different from dissolution inhibition photoresists. EP 0 227 487 A discloses a positive-working radiation-sensitive resin composition comprised of a 1,2-quinonediazide and an alkali soluble novolac resin produced from condensation of two types of phenolic compounds. This document does not disclose photoresists comprised of poly(vinyl phenols), the subject of the present invention. JP-A60 017 739 discloses a negative-working photoresist comprised of hydroxy-styrene polymer and a photosensitive azide compound. Since this document discloses negative photoresists, it is not particularly relevant to the positive-working photoresists of the present invention. EP 0190799 A discloses the use of a photosensitive dissolution inhibitor with a binder such as poly(2,4-dihydroxystyrene) in a positive-working photoresist. This exemplified binder (poly(2,4-dihydroxystyrene)) in this disclosure does not possess vicinal hydroxyl substitution and thereby teaches away from the present invention.
Poly(hydroxystyrene) resins [(poly(vinyl phenol) resins] have heretofore not been generally used extensively in microlithography applications primarily because of their low inhibitability. Therefore, there exists a need in the art for improved poly(hydroxystyrene) and other resins with increased inhibitability.